Mathematical modelling in epizootology makes it possible to forecast the occurrence and spreading of infection, to learn the main factors of the origin and spread of infection, or to test hypotheses on these factors. Therefore epizootological models must be correct from the biological and mathematical view-point. They should not contradict to experimental facts, must be sufficiently sensitive to important factors, and must be able to approximate real epizootological phenomena and processes. Examples of the construction of simple deterministic and stochastic models of exogenous infections whose etiological agents meet the conditions of Henle-Koch's postulates are used for demonstrating the basic approaches to the use of mathematical models for the evaluation of epizootological analyses and programmes of infection control.